This invention relates to stereophonic signal demodulation circuits, and more particularly to an FM multiplex (MPX) stereophonic signal demodulation circuit.
Heretofore, a double balanced type demodulation circuit comprising differential amplifiers has been employed as an FM MPX stereophonic signal demodulation circuit. In this conventional demodulation circuit, the differential amplifiers are cascade-connected in a two-stage state. Therefore, the power supply utilization factor is low, but it is impossible to increase the dynamic range. Furthermore, since the signal distortions of the differential transistors in the stages are superposed, it is difficult to reduce the distortion factor. In addition, because of the unbalance in characteristics of the differential transistors, distortion occurs, and the right and left channel signals have differences in level. Moreover, for muting operation in the selection of an FM broadcast station, it is necessary to provide a particular muting circuit in the rear stage of the double balanced type demodulation circuit.
In order to eliminate the above-mentioned drawbacks accompanying in the double balanced type demodulation circuit, a switching type demodulation circuit has been formulated which employs a chopper type switching circuit to thereby separate the information into right and left channels. In this prior art chopper type switching circuit, however, switching pulses, i.e., the leak of the sub carrier, appear on the output terminal, thereby causing the signal to noise ratio to be degraded. Another drawback in this switching circuit is that the level of the output signal when receiving monaural and stereophonic signals are not equal.